1. Field of the Invention
This invention relates to a platinum complex for an organic light-emitting diode, more particularly to a phosphorescent platinum complex having a dianionic bidentate ligand and a bidentate, nitrogen-containing heteroaromatic ligand.
2. Description of the Related Art
An organic electroluminescence device normally includes an organic light-emitting diode (OLED) and a driving element. An OLED includes an anode, a cathode, and an organic layer (i.e., light-emitting layer) disposed therebetween for emitting light when a voltage is applied between the anode and the cathode. Generally, the organic layer is made from a phosphorescent material, since phosphorescent material is able to emit light as a consequence of decaying of excitons from triplet state to ground state. As such, it is desirable to improve the phosphorescent organic layer so as to enhance the light-emitting efficiency of OLEDs. The inventor of the present invention previously disclosed in U.S. Pat. No. 7,002,013 a phosphorescent platinum complex with the following general formula (A):

where
X1 and X2 are each independently C or N,
X1 can also be located at another position of the hexagonal ring, when X1 is N,
Ra is H, C1-C8 alkyl, or C1-C4 perfluoroalkyl, Rb is H, Ra and Rb together are C4-C8 alkylene, or Ra and Rb together are bridged carbocyclic C4-C12 alkylene, when X2 is C,
Ra is H, C1-C8 alkyl, or C1-C4 perfluoroalkyl, and Rb is omitted, when X2 is N,
Rg is H or methyl, and Rh is omitted, when X1 is N, and
Rg is H or methyl, and Rh is H or methyl, or Rg and Rh together are
when X1 is C.
The platinum complex is electroneutral, and the two ligands thereof are the same. Thus, it is difficult to fine-tune an energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for tuning an emission wavelength.